Conventionally, a position determination system meeting IS (Interim Standard) 801 is one of the position determination systems used to determine a position of a mobile terminal, e.g., a cell phone. In the system meeting IS 801, the mobile terminal starts base station information collection when the position determination is requested by a user, and then transmits the collected base station information to a position information server (position server).
The position server receives the base station information and determines an approximate position of the mobile terminal based on the base station information. The position server then transmits assist information to the mobile terminal for capturing appropriate GPS (Global Positioning System) signals of GPS satellites. The mobile terminal captures the GPS signals based on the assist information and transmits position information acquired from the GPS signals to the position server. The position server then calculates an accurate position of the mobile terminal based on the position information.
In this manner, the mobile terminal can avoid repeatedly capturing GPS signals and determining the appropriate GPS signals and therefore can reduce its calculation load. Therefore, a preparation term for starting position determination can be shortened.
The base station information collection is described with reference to FIGS. 10 to 12. In FIG. 10, when a position determination start instruction (signal) is transmitted by a user or the like, a PPP (Point to Point Protocol) connection and a TCP (Transmission Control Protocol) connection are still not performed between the mobile station and the position server. Upon receiving the position determination start instruction, a position determination management task transmits a base station information collection start request (signal) to a search task in the mobile terminal. The PPP connection is then executed and the TCP connection is further executed. The position determination management task transmits a base station information collection result request to the search task. Accordingly, since a term between the base station information collection start request and the base station information collection result request is long, it is enough time to collect the base station information. Therefore, accurate base station information can be transmitted to the position server.
However, as shown in FIG. 11, when the PPP connection has already been performed before the position determination start instruction is generated by a user, the TCP connection is only performed thereafter. Accordingly, since the term between the base station information collection start request and the base station information collection result request is too short, it is not enough time to collect the base station information. Therefore, accurate base station information cannot be transmitted to the position server.
In this condition, the position server calculates the approximate position of the mobile terminal based on inaccurate base station information. Therefore, the position server transmits inaccurate assist information to the mobile terminal, and the mobile terminal cannot capture appropriate GPS signals. As a result, because the position server cannot calculate an accurate position of the mobile terminal, the accuracy of the position determination decreases.
Also, as shown in FIG. 12, when a communication between the mobile terminal and the position server rebuilds from a dormant state due to a hard handoff therebetween, the same disadvantage mentioned above may occur. That is, the search task transmits a base station information collection stop instruction to the position determination management task at the hard handoff, and thereafter the position determination management task again transmits the base station information collection result request.
Accordingly, since the term between the base station information collection start request and the base station information collection-result request is too short, it is not enough time to collect the base station information. Therefore, accurate base station information cannot be transmitted to the position server, and the same disadvantage mentioned above occurs.